Sunscreens: Making Sense of Sunscreen Products (cont.)

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However, it is a common mistake to assume that the duration of effectiveness of a sunscreen can be calculated simply by multiplying the SPF by the length of time it takes for him or her to suffer a burn without sunscreen, because the amount of sun exposure a person receives is dependent upon more than just the length of time spent in the sun. The amount of sun exposure depends upon a number of factors, including the length of exposure, time of day, season, geographic location, and weather conditions.

The sun's rays contain different wavelengths of ultraviolet (UV) light. The two types of UV rays that pass through the earth's atmosphere and cause damage to the skin are UVB and UVA. UVB rays are the primary cause of sunburn and affect the outer layer of skin.

The strength of UVB radiation depends upon the time of day, season of the year, and geographic location. UVB rays are most intense from 10 a.m. to 2 p.m. and are stronger in summer, at higher altitudes, and closer to the equator.

Unlike UVB rays, which do not penetrate glass, UVA rays can travel through window glass and damage the deeper layers of the skin. Both UVA and UVB light contribute to age-related changes in the skin such as wrinkles, freckles, age spots, and prominent blood vessels. Both UVA and UVB exposure raise the risk of skin cancer. A major limitation of the SPF value is that these numbers measure protection against sunburn caused by ultraviolet B (UVB) radiation. Therefore, SPF values only indicate a sunscreen's UVB protection and do not provide any information on the product's effectiveness in blocking the UVA rays that contribute to the development of skin cancers.

Sunscreens can be broadly classified into two categories: chemical sunscreens and physical sunscreens. Chemical sunscreens absorb UV radiation while physical sunscreens act by physically blocking it. Chemical sunscreens can be UVA or UVB absorbers. Many sunscreens have a combination of ingredients and may contain both physical and chemical sunscreens.

Physical sunscreens are good blockers of both UVA and UVB radiation. The two most common physical blockers of UV radiation are titanium dioxide and zinc oxide. Examples of chemical sunscreens include the following:

  • PABA (para-aminobenzoic acid): Rarely found in modern preparations, PABA was an early chemical sunscreen that often induced sensitivity reactions.
  • PABA esters (glyceryl, padimate A, and padimate O): These newer preparations have fewer side effects than the original PABA.
  • Salicylates (homosalate, octyl salicylate)
  • Cinnamates (cinoxate, octyl methoxycinnamate, or octocrylene): Octocrylene is a cinnamate with both UVA- and UVB-absorbing properties.
  • Benzophenones: These can absorb both UVA and UVB rays.
  • Ecamsule (Mexoryl) is a potent UVA-blocking compound.
Medically Reviewed by a Doctor on 12/1/2014